1. Field of the Invention
The present invention relates to a device for measuring at least one parameter of a flowing fluid medium, e.g., a fluid medium flowing through a flow pipe.
2. Description of Related Art
In many processes, for example in the field of process engineering, chemistry and mechanical engineering, fluid media, in particular gas masses (such as an air mass) having certain properties (for example, temperature, pressure, flow velocity, mass flow, volumetric flow, etc.) must be supplied in a defined manner. These processes include, in particular, combustion processes which run under regulated conditions.
An important application example is the combustion of fuel in internal combustion engines of motor vehicles, in particular having subsequent catalytic exhaust gas treatment, in which a certain air mass per time unit (air mass flow) must be supplied in a regulated manner. Different types of sensors are used to measure the air mass flow rate. One sensor type known from the related art is the so-called hot film air mass meter (HFM), which is described, for example, in a specific embodiment in published German patent application document DE 196 01 791 A1. A sensor chip, which has a thin sensor diaphragm, for example a silicon sensor chip, is usually used in hot film air mass meters of this type. At least one heating resistor, which is surrounded by two or more temperature measuring resistors (temperature sensors), is typically situated on the sensor diaphragm. In an air flow which is conducted over the diaphragm, the temperature distribution changes, which, in turn, is detectable by the temperature measuring resistors and may be evaluated with the aid of a control and evaluation circuit. For example, an air mass flow may be determined from a difference in the resistance of the temperature measuring resistors. Various other variants of this sensor type are known from the related art. However, the present invention is not limited to the sensor type of the hot film air mass meter described, but instead may be used, in principle, for most types of sensors which are used as permanently installed sensors or as plug-in sensors in a flowing medium.
The disadvantage of the plug-in sensor designs described in the related art is that the described plug-in sensors in many cases cause problems relating to a pressure drop in the intake tract due to a flow resistance. In particular, this means that the signal reproducibility of the signals of sensors of this type is not optimal.
Many sensors, in particular hot film air mass meters, are in practice equipped with a grating or a grating combination. These gratings may be integrated, for example, into a flow pipe and are usually located a few centimeters upstream from the plug-in sensor or other sensor in the flow and have the function of equalizing the velocity profile in the flow pipe. Furthermore, gratings of this type have the function of removing a swirl, which is occasionally present, from the flow. The equalizing effect of the grating is achieved by its braking effect on the flow. At the same time, a small-scale turbulence is generated, which mixes fast and slow fluid and thereby contributes to a velocity compensation over the entire pipe cross section. This achieves the fact that the characteristic of the sensor (for example, a correlation between air mass and output frequency or output voltage) is nearly independent of the velocity profile of the in-flowing air.
An example of a sensor system having a grating is described in published German patent application document DE 199 42 502 A1. Herein, a measuring probe is situated in a line or a pipe body downstream from a protective grating which induces a deflection of the medium flow and deflects liquid particles. Longitudinal ribs or turbulators, which prevent uncontrolled vortex separation, may be provided downstream from the protective grating.
Many sensor systems, in particular air mass sensors, in combination with conventional gratings, for example plastic gratings having a mesh width between 4 mm and 7 mm and a mesh depth of 5 mm to 10 mm, unfavorably demonstrate comparatively poor reproducible characteristics in certain air mass ranges. Among other things, this problem is caused by laminar-turbulent transition of the flow at the individual grating bars. Since the transition response is highly sensitive to the most minute changes in the conditions, such as temperature, geometry, flow velocity or similar conditions, and these parameters are, as a rule, not entirely constantly present over the cross section of the flow tube, the flow is not precisely turbulent everywhere at the same time. This results in the fact that air mass areas exist where the flow state (laminar or turbulent) of the flow of the fluid medium downstream from the grating is not precisely defined and is comparatively difficult to reproduce. This uncertainty is reflected in the reproducibility of the characteristic of the sensor element.